Properly support regions in overlap for negative impls

[spastorino]

The following code should not compile but it does. This was noted in rust-lang/rust#93175 where we know we are doing predicate_must_hold_modulo_regions but we shouldn't exactly do that.

trait Foo { }
impl !Foo for &'static () { }

trait Bar { }
#[rustc_strict_coherence]
impl<T: Foo> Bar for T { }
#[rustc_strict_coherence]
impl Bar for &() { }

[spastorino]

I think we can close this one now that's merged. cc @nikomatsakis